Long interspersed element-1 (L1) is an autonomous non-long terminal repeat retrotransposon that has parasitized the human genome for millions of years. L1 has shaped the evolution of the human genome through a copy-and-paste mobilization of itself (Moran et al., Cell 1996, 87(5):917-927) as well as the short interspersed element (SINE) Alu, SINE-VNTR-Alu elements (SVA), and processed cellular transcripts (Dewannieux et al., Nat Genet 2003, 35(1):41-48; Ostertag et al., Amer J Human Genetics 2003, 73(6):1444-1451, Esnault et al., Nat Genet 2000, 24(4):363-367). Functional full-length L1 transcripts contain two open reading frames (ORFs) encoding ORF1 and ORF2 proteins (ORF1p and ORF2p, respectively), as seen in FIG. 1A. These L1 proteins exhibit cis-preference for their encoding L1 mRNA (Kulpa and Moran, Human Mol Genetics 2005, 14(21):3237-3248; Wei et al., Mol Cell Biol 2001, 21(4):1429-1439, Kolosha and Martin, Proc Natl Acad Sci (USA) 1997, 94(19):10155-10160), and are utilized in trans by the Alu and SVA elements (Dewannieux, supra, Ostertag, supra, Wallace et al., Gene 2008, 419(1-2):1-6). L1, Alu, and SVA form ribonucleoprotein (RNP) particles that reach the nucleus to complete their replication cycles by integrating in the host genome via a process of target-primed reverse transcription (Luan et al., Cell 1993, 72(4):595-605, Cost et al., EMBO J, 2002, 21(21):5899-5910). This copy-and-paste process has produced approximately 500,000 L1 loci, accounting for about 17% of the human genome, and over 1,000,000 copies of Alu, which comprise about 11% of the human genome (Lander et al., Nature 2001, 409(6822):860-921). The majority of the L1 loci are 5′ truncated with about 80-100 full-length L1 copies demonstrated to be retrotranspositionally active (Konkel et al., Gene 2007, 390(1-2):28-38; Beck et al., Cell 2010, 141(7):1159-1170; Huang et al., Cell 2010, 141(7):1171-1182; Ewing and, Kazazian, Genome Res, 2011, 21(6):985-990; Brouha et al., Proc Natl Acad Sci (USA), 2003, 100(9):5280-5285).
L1 proteins are produced from the full-length L1 mRNA with significantly different efficiencies, mostly owing to the unconventional translation from the bicistronic L1 mRNA, shown in FIG. 1A (Basame et al., J Mol Biol, 2006, 357(2):351-357; Khazina et al., Nat Struct Mol Biol 2011, 18(9):1006-1014; Callahan et al., Nucl Acids Res, 2012, 40(2):813-827; Taylor et al., Cell 2013, 155(5):1034-1048). Detection of both L1-encoded proteins is important for understanding L1 biology because each plays a critical but different role in the L1 replication cycle. The human ORF2 protein (“ORF2p”) is a 149 kilodalton (kDa) protein with three annotated domains: an N-terminal endonuclease (EN) domain, a reverse transcriptase (RT) domain, and a C-terminal domain with putative RNA binding activity (Feng et al., Cell 1996, 87(5):905-916; Xiong and Eickbush, EMBO J, 1990, 9(10):3353; Fanning et al., Biochimica et Biophysica Acta, 1987, 910(3):203-212; Piskareva et al., FEBS Open Bio 2013, 3:433-437). Human and mouse L1 ORF2 proteins exhibit a high degree of sequence homology and conservation of function, making findings in mouse model systems biologically relevant to the replication cycle of the human L1 (Alisch et al., Genes & Dev, 2006, 20(2):210-224; Li et al., Nucl Acids Res, 2006, 34(3):853-864). Much has been learned about ORF2p function in vitro and in mammalian cells using overexpressed tagged ORF2 proteins and polyclonal anti-ORF2p antibodies (Ergun et al., J Biol Chem, 2004, 279(26):27753-27763; Kines et al., Nucl Acids Res, 2014, 42(16):10488-502; Goodier et al., Human Mol Genetics, 2004, 13(10):1041-1048; Doucet et al., PLoS Genet, 2010, 6(10):e1001150).
Monoclonal antibodies that can specifically recognize human ORF2 protein would be useful to study ORF2p expression and activity. They would also be useful in understanding ORF2p impact on host genome instability and the consequences of ORF2p activity on human health. Unfortunately, despite efforts to develop monoclonal antibodies that recognize ORF2 protein, no such antibody is commercially available, nor has any group reported success in producing one.